1. Field of the Invention
The present invention relates to an image forming apparatus with an application unit for applying a lubricant to an image support.
2. Description of Related Art
In an electrographic image forming apparatus, as shown in FIG. 7, the circumferential surface of an image support 102 is uniformly charged by a charging unit 101, and then irradiated with an optical beam so that an electrostatic latent image is formed thereon. Thereafter, in the image forming apparatus, a developing unit 103 develops the electrostatic latent image on the image support 102, thereby forming a toner image. Moreover, in the image forming apparatus, the toner image formed on the image support 102 is transferred onto a sheet material such as paper, and then the transferred toner image is fused and fixed on the sheet material. The image forming apparatus also includes a cleaning unit 104 provided so as to contact the image support 102 for the purpose of removing toner remaining on the image support 102 without being transferred onto the sheet material (hereinafter, such toner will be referred to below as untransferred toner). The cleaning unit 104 includes a cleaning blade 105 made of strips of polyurethane. The cleaning blade 105 is pressed against the image support 102, thereby scraping off untransferred toner.
In recent years, to reduce downtime of the image forming apparatus, there is demand for extended service lives of the image support 102 and the cleaning blade 105. A factor that limits the service lives is an amount of abrasion. In an abrasion reduction technique in practical use, an application brush 107 is used to apply a solid lubricant 106 onto the circumferential surface of the image support 102, thereby forming a film of solid lubricant (referred to below as a lubricant film) thereon.
However, reducing the amount of abrasion of the image support 102 means reducing the force of the application brush 107 scrubbing the circumferential surface of the image support 102, resulting in other issues. The most critical of all is an image defect. Specifically, charged products (such as O3 and NOx) generated by the charging unit 101 adhere to the image support 102. The adhering charged products might be left unscrubbed when the lubricant film weakens the scrubbing force of the image support 102. The remaining charged products might lower the surface resistance of the image support 102, making the electrostatic latent image on the image support 102 defective. Accordingly, for example, a resultant print includes an image with blurry edges or a half-tone image with white spots. Such image noise is included in the image defect. Note that in the case where the amount of abrasion of the image support 102 is high, charged products are scrubbed together with the film on the surface of the image support 102, so that no image defect is caused.
The mechanism of the image defect will now be described in more detail. In the following, the percentage of applied lubricant is a value obtained by dividing the amount of solid lubricant 106 applied to the image support 102 by the amount of lubricant scraped by the application brush 107. In the configuration of FIG. 7, both the image support 102 and the application brush 107 rotate counterclockwise, and the percentage of applied lubricant is less than 50%, so that there are numerous lubricant particles 108 (represented by small circles) where the image support 102 and the application brush 107 contact each other, as shown in FIG. 8. The lubricant particles 108 have a low friction coefficient, and therefore reduce a frictional force generated between the image support 102 and the application brush 107. As a result, the application brush 107 cannot achieve the effect of scrubbing the circumferential surface of the image support 102, so that charged products are not removed from the circumferential surface of the image support 102, causing an image defect.
An image forming apparatus capable of reducing the image defect is described in, for example, Japanese Patent Laid-Open Publication No. 2001-265185. This image forming apparatus includes a lubricity providing member, an application roller, and a friction coefficient adjusting member. The lubricity providing member and the friction coefficient adjusting member are arranged in this order along the rotational direction of the application roller. The lubricity providing member supplies a lubricant to the application roller. The application roller with the lubricant contacts the friction coefficient adjusting member, thereby removing excess lubricant from the application roller. Thereafter, the application roller applies the lubricant to the image support.
Incidentally, the lubricant film on the image support adsorbs charged products, and therefore, to prevent image defects, it is important to scrub the lubricant film with charged product adsorbed thereon, and form a new lubricant film without any charged product being adsorbed thereon.
However, simply adjusting the amount of lubricant to be supplied as in the approach described in Japanese Patent Laid-Open Publication No. 2001-265185 might not successfully remove the lubricant film with charged product adsorbed thereon, so that the lubricant film with charged product adsorbed thereon possibly remains on the image support, causing an image defect.